This means Lightroom cannot find the photos on one of your hard disks, because you (or some program) have moved or renamed or deleted it outside of Lightroom. There is probably an exclamation point icon on the photo, as shown in Figure 2 here: Adobe Lightroom - Find moved or missing files and folders CLick on the exclamation point icon and direct Lightroom to the proper location/file name of the photo.

It could also mean the disk with the photo is unplugged, in which case you would plug the disk in.